In a multimedia age, optical recording media such as compact disc recordable (CD-R, a write-once memory using compact disc); and digital versatile disc (DVD-R, a write-once memory using digital video disc), have been highlighted. Optical recording media can be classified roughly into inorganic optical recording media which have recording layers composed of inorganic substances such as tellurium, selenium, rhodium, carbon, or carbon sulfide; and organic optical recording media which have recording layers composed of light absorbents containing organic dye compounds.
Among these optical recording media, organic media are usually prepared by dissolving a polymethine dye in an organic solvent such as 2,2,3,3-tetrafluoro-1-propanol (abbreviated as “TFP” hereinafter), coating the solution onto the surface of a polycarbonate substrate, drying the solution to form a recording layer, and sequentially attaching closely a reflection layer made of a metal such as gold, silver or copper and a protective layer made of an ultraviolet ray hardening resin onto the surface of the recording layer. When compared with inorganic optical recording media, organic optical recording media have the drawback that their recording layers may be easily changed by exposure to light such as reading- and natural light, but have the merit that they can be manufactured at a lower cost because their recording layers can be formed by preparing solutions of light absorbents and directly coating the solutions onto the surface of substrates. Also, organic optical recording media are now becoming the predominant low-cost optical recording media because of the merits that they are mainly composed of organic substances so that they are substantially free of corrosion even when contacted with moisture or sea water; and because information, which is stored in optical recording media in a prescribed format, can be read out using a commercialized reader using thermal deformation type optical recording media, a kind of organic optical recording media.
What is urgently required of organic optical recording media is to increase their recording capacity to suit this multimedia age. The research for such an increment now eagerly continued in this field is to shorten the wavelength of 635-650 nm now used as a writing light to a wavelength of 450 nm or less to increase the recording capacity per one side to a level from 4.7 giga bytes (GB) to 15 GB or higher. The optical recording media with such an increased capacity can record six hours of moving images in quality equivalent to standard TV and just record two hours of moving images in quality equivalent to high-quality TV. However, most of the organic dye compounds now used in optical recording media are not applicable to laser beams with a wavelength of 450 nm or less, and therefore such organic dye compounds could not fulfill the need for high storage density required in many fields.